Mail and wire fraud are covered in 18 U.S.C. secs 1341 and 1343. “Theft of honest services” under 18 U.S.C. sec 1346 is a section under mail and wire fraud. In McNally v. U.S. the Supreme Court rejected the idea that theft of honest services was different offense that traditional mail fraud. In 1988 the U.S. Congress amended 1346 to overturn McNally.
In Skilling v. U.S., the United States Supreme Court concidered if sec. 1346 was unconsitutionally vague. The United States Supreme Court decided that it was not vague as to bribes and kickbacks. But, 1346 should not cover undisclosed self dealing by a public official or private employee and, therefore Skilling did not commit honsest-services fraud.
For a fuller explination without having to read the opinion, “Jeff Skillings’s Constitutional Challenge to ‘Theft of Honest Services'” by David McAtee II and Jason E. Wright is a great article.